I couldn't find any reference to the spelling reform in 1996, but I found that the word Wage (Scale) was changed to Waage in 1927 to separate it from the word Wagen (Car, Cart, Coach). I'm sure, the word wagerecht was changed at the same time to waagerecht. I found it for example in the big BF110B-manual on page 24.

My conclusion is, that this is bad spelling but good engineering. They had letters with a certain size and there was not enough room for the correct word. So they used the wrong spelling.

It is possible, that both version were allowed at that time.

That are the results when bavarians have to write in german!
Could also bee an bad translation from Wagrecht (bavarian spoken: woagrecht) to German Waagerecht.